I just purchased a 2012 Yukon XL Denali and while reading through the manual I came across the section discussing fuel. I am a bit confused, however, as to what I should be using. There is a section that states that “Vehicles that have a FlexFuel badge and a yellow fuel cap (mine has both) can use either unleaded gasoline or… E85” but the next section on recommended fuel states that “premium unleaded gasoline with a posted octane rating of 91 or higher” be used. I’m not clear; I know I can use E85 but should I use premium fuel otherwise? If the engine is designed for E85 wouldn’t it be able to handle normal 87 octane unleaded just fine?

E85 has higher octane than regular, so it’s possible premium is required. But I would have thought that you could use regular, just like non-flexfuel Yukons. Would you give us the whole quote about “premium unleaded gasoline”? Anything after that part you gave us?

Fuel E85 (85% Ethanol)
Vehicles that have a FlexFuel badge and a yellow fuel cap can use either unleaded gasoline or ethanol fuel containing up to 85% ethanol (E85). For all other vehicles, use on ly the unleaded gasoline described under Recommended Fuel on page 9-52.

Recommended Fuel
Use premium unleaded gasoline with a posted octane rating of 91 or higher. you can also use regular unleaded gasoline rated at 87 octane or higher, but the vehicle’s acceleration could be slightly reduced, and a slight audible knocking noise, commonly referred to as a spark knock, might be heard. If the octane is less than 87, you might notice a heavy knocking noise when you drive. If this occurs, use a gasoline rated at 87 octane or higher as soon as possible. Otherwise, you could damage the engine. If heavy knocking is heard when using gasoline rated at 91 or higher, the engine needs service.

Didn’t you just answer your own question here? Use either E85 or 91-octane regular gas, whichever is cheaper per mile (probably the regular gas). You could try to get away with 87-octane regular gas, but most likely your fuel economy will be reduced when the engine timing changes to compensate for the lower octane, canceling out the advantage of its lower cost, not to mention that you run the risk of engine damage if the knock sensor ever fails.

OK, so this is a case where premium is recommended, but not required. I’d try both and see if I noticed a difference in performance and mpgs (keep close track of miles and gallons, the difference might be small). I’d skip the E85, you’ll take about a 30% hit in mpgs and range.

I don’t know where this car was manufactured, but users manuals are usually poorly written.

When there are ambiguities, and there are many, you should take the conservative approach (premium fuel in this case) until you can ask a dealer for an interpretation.

Although that is usually useless, but if you get it in writing at least you are covered. HA, good luck with that.

Just something as simple as oil change intervals is pretty indeterminate, sorry to say… That is because there are two different intervals stated, ordinary and severe, and the definition of “severe” is very vague. For example, if you take the definition in my manual, most drivers are in the “severe” category.

I guess I see your point about the exact way they wrote this, but I think they intended the “Recommended Fuel” section to apply to all vehicles. I can’t think of any reason the FlexFuel vehicle would have different octane requirements.

Looks pretty straightforward to me. The manual allows E85 but recommends premium.
Clearly it is also warning you AGAINST anything else.

They’ve specifically warned you that the use of anything else “could damage the engine”. The “heavy knocking” they’re warning you against will be from the fuel self-igniting before the piston reaches the point where the spark plug is supposed to start the explosion. In short, the explosion will happen while the piston is still coming up the cylinder in its compression stroke. That is NOT GOOD! You’ll have the force of the piston coming up, being pushed by rotational inertia AND by other cylinders firing and pushing it up, banging against the force of the explosion. It could blow a hole in the piston, damage a rod bearing, or even bend or break a rod.